Optical instruments of the kind wherein a component region is masked in order that the remaining region can be seen with greater clarity are found in many areas. The masking is especially important where an intense self-radiator is present which blankets non-masked weaker radiating peripheral regions. This problem is especially significant in coronagraphs with which the corona of the sun is viewed.
The masking out of component regions as such has been utilized for a long time. However, in known apparatus, lenses are used ahead of the location where the masking is done and occult discs are utilized for masking. However, if very low luminescence is to be viewed adjacent intense radiators, very high requirements must be imposed on the imaging system with respect to a low stray radiation. Individual lenses, however, lead to a deterioration of the imaging because of known lens aberrations.
The lens aberrations can be minimized by utilizing several lenses. However, this leads to a reduction in light in dependence upon the number of lenses because of the lens material and to an increase in the stray radiation caused by the surface roughness of each lens. These conditions together effect a reduction of the overall contrast. Accordingly, with lens systems, either a good image is obtained or little stray radiation.
The use of occulting discs is a second disturbing factor which together with the utilization of lenses limits the minimization of stray radiation.
Coronagraphs belong to those apparatus which impose the highest requirements with respect to the foregoing. With coronagraphs, the masking out of the central region has up to now been achieved with a mirrored occult disc on a field lens. However, since a lens generates stray radiation at two surfaces and in its interior, limits are here imposed on the reduction of stray light.